They are converting just the chatwindow widget into a library, making it possible for other applications to make use of it (as an example, KDE games could use it for an in-game chat).
The complex part of the work is not really turning the chatwindow itself into a library, the problem is that it is making heavilly usage of some Kopete classes (like Kopete::MetaContact, as an example), so making it reusable by other applications requires some changes to make the participants info more generic.

This if I understand correctly what you were trying to mean by “integrating with libpurple”